Using tabular conflicts with the massaging done by beamer, whereas a more primitive approach is successful, in the sense of using primitive commands such as \vbox, \hbox or \halign that won't be untimely expanded during the processing by beamer. The optional argument to \theauthor can be used to set the width of the image (default 3cm).

Assuming you're okay with essentially creating a custom title page, you can use the \columns environment to create blocks for your authors. Modifying the \theauthor command to include an extra #1 as shown will place the author name under each picture. Alternatively you can add an extra argument to the command for more flexibility.

What it looks like

The depicted people are Emmy Noether and George Stokes. I just wanted a pair of public domain photos that no one would care about me using.